In the Congress, the abortion issue has surfaced this session in some 20 different legislative areas - ranging from banning abortions in US military hospitals to elimination of abortion coverage in federal employees' health plans.

But the partial-birth or D&amp;X abortion ban stands alone as the most significant innovation in anti-abortion strategy in recent years. The bill, expected to pass in the Senate yesterday, represents the first effort in Congress to ban a specific abortion procedure since abortion was proclaimed a constitutional right by the Supreme Court in its 1973 Roe v. Wade decision.

Why has it taken abortion foes in Congress so long to try to outlaw a particular method? And why has the arena of the entire abortion battle shifted from the Supreme Court to the legislatures and the streets?

The answer lies within the Supreme Court itself.

In 1989 and again in 1992, the high court handed down key rulings that opened the door to abortion-rights challengers to restrict access to the procedure. The first ruling, a Missouri case called Webster v. Reproductive Health Services, let stand a law carrying numerous limits, including a prohibition on abortion in state facilities except to save the life of the mother. For the first time, four justices voted solidly against Roe.

Then in 1992, in a Pennsylvania case called Planned Parenthood v. Casey, the court upheld further restrictions, including one requiring women to observe a waiting period before having an abortion. The Casey ruling reaffirmed the central tenet of Roe, that abortion before viability may not be banned. Viability is the point at which a fetus can survive outside the womb.

But ''the most important fallout from Casey is that it established a lower standard of scrutiny'' for laws seeking to limit abortion rights, says Janet Crepps, a lawyer at the New York-based Center for Reproductive Law and Policy. Roe had established a requirement of ''strict scrutiny''; in Casey, Justice Sandra Day O'Connor put in place a lower standard requiring that laws not place an ''undue burden'' on access to abortion.

''Webster opened the door ajar, and Casey pushed it open wider,'' says Kate Michelman, president of the National Abortion and Reproductive Rights Action League.

Since those two cases, anti-abortion legislators have sought to put in place laws carrying the restrictions of Webster and Casey - and other laws that push the envelope of abortion limits and could eventually reach the Supreme Court as opportunities to overturn Roe outright. Abortion opponents want to see how much they can restrict under the new ''undue burden'' standard, which remains unclear.

The inspiration for the bill in Washington banning D&amp;X (dilatation and extraction) abortions originated in Ohio, where similar state legislation is now being challenged in court. The Ohio bill would ban the D&amp;X procedure, a rarely used technique typically performed late in a pregnancy when the mother's life or health are threatened or the child is diagnosed with severe abnormalities. Of the 1.5 million abortions performed annually in the US, several hundred are D&amp;X.

Abortion foes call it a partial-birth abortion because the procedure entails partial delivery of an intact fetus. Abortion-rights supporters reject that name as inflammatory, adding that the term is not recognized in medical circles.

The bottom line, though, is that before Webster and Casey, the Supreme Court had made clear it would not allow efforts to restrict abortion techniques. In 1976, in another Missouri case, called Planned Parenthood v. Danforth, the high court rejected the state's ban on another abortion procedure called saline amniocentesis. But with a new Republican-controlled Congress, where abortion foes hold a majority of House seats, a law to ban a procedure nationwide will now for the first time reach the president's desk (though he is unlikely to sign it).

Meanwhile, the Ohio bill is heading for the Supreme Court. So far, abortion-rights supporters have blocked enactment of the law with a temporary restraining order. On Dec. 1, a US district judge in Ohio stated that the law may be unconstitutional, citing two possible grounds: First, that ''it appears to be vague'' and, second, that it may ''create a chilling effect that would prevent physicians from performing post-viable abortions where, in their own best judgment, an abortion is necessary to preserve the life or health of the mother.''